When flat metallic material is drawn into a cup shape or it is blanked and drawn simultaneously, the material must be confined between a pressure pad and a draw die to prevent wrinkles. Pressure is generally exerted pneumatically and the amount of pressure exerted on the edges of the blank must be sufficient to prevent wrinkling. If the pressure is too high the metal will not be permitted to move and the draw punch will simply punch a hole through the metal blank. Wrinkling occurs almost immediately at the start of the drawing operation if the clamping pressure between the pressure pad and the draw die is sufficient. Once the direct pressure for wrinkle prevention is established and the depth of draw progresses, the area of undrawn metal must sustain all of the originally established clamping pressure because of the progressive decrease in blank size.
During metal drawing operations a phenomenon generally known as "earing" occurs. This is generally manifested by the formation of ears at approximately 45.degree. to the direction of rolling of the material from which the blank is punched. The explanation for this phenomenon is not clear but in general metal at 0.degree. and 90.degree. of the rolling direction will drawn uniformly with respect to the rolling direction, whereas metal at 45.degree. points will slip along elongated grains. Due to this slippage it does not thicken as much as the metal at the 0.degree. and 90.degree. points and therefore the ears are produced at 45.degree. points. These ears will be thinned and be pinched off and result in contamination of the tooling and must be removed.
Various attempts have been made to avoid the problems of earing and wrinkling and have not been uniformly successful.
According to the present invention, the ram of the press which carries the draw punch assembly and which it is arranged to reciprocate for performing the draw, is provided with a cylinder and a piston therein which piston, through a plurality of pressure pins, bears on a pressure pad.
The bed of the press carries the draw die and if desired also a blanking die and a blanking punch. A controlled air pressure is applied to the upper side of the piston in the ram and maintained at a constant value. Therefore, as the ram descends and the pressure pad bears against the blank resting on the draw die and the draw punch begins to enter the draw die to perform the cup drawing operation, the pressure pad resting on the draw die through the pressure pins bearing against the underside of the piston forces the piston up against the controlled constant air pressure and thus a constant pressure is maintained on the edge of the blank.
The bed also carries a blanking die and a blanking punch and the pressure pad has a flange arranged to bear against the blanking punch as the pressure pad enters the blanking punch to bear against the metal to be drawn. The flange on the pressure pad bearing on the blanking punch causes the blanking punch to blank out the material resting on the draw die in cooperation with the blanking die.
The metal sheared off by the blanking punch and die is pinched between the blanking punch and a movable stripper and a movable stripper is provided with a spacer so that the pressure pad in bearing against the blanking punch cannot approach the draw die beyond a predetermined point, thereby maintaining a predetermined gap between the pressure pad and draw die.
By virtue of the maintenance of the predetermined pressure pad-draw die gap and maintenance of the predetermined air pressure on top of the piston in the ram, thinning and clipping of ears and wrinkling are prevented.